10.2478/v10285-012-0049-5 Journal of Landscape Ecology (2012), Vol: 5 / No. 1. GEOGRAPHICAL ASSESSMENT OF FACTORS FOR SASA EXPANSION IN THE SAROBETSU MIRE, JAPAN MASAYUKI TAKADA*, TAKASHI INOUE**, YOSHIO MISHIMA**, HIROKO FUJITA***, TAKASHI HIRANO**, YOSHIYASU FUJIMURA*** *Hosei University, 2-17-1, Fujimi, Chiyoda-ku, Tokyo 102-8160, Japan, e-mail: [email protected] **Hokkaido University, Graduate School of Agriculture, North 9, West 9, Kita-ku, Sapporo 060-8589, Japan ***Hokkaido University, Botanic Garden, Field Science Centre for Northern Biosphere, North 3, West 8, Chuo-ku, Sapporo 060-0003, Japan Received: 30th August 2010, Accepted: 15th February 2012 ABSTRACT To determine the factors that promote the expansion of dwarf bamboo (Sasa palmate), an indicator of mesic vegetation, into bogs, a landscape-based approach was used to assess the geographical factors that are associated with, and contribute to, the propagation of Sasa in the Sarobetsu Mire, northern Japan. Using the “Sasa frontline” data obtained from aerial photographs taken during 2 different periods, the area expanded by Sasa in the past 23 years was determined. Next, distribution maps associated with geographical parameters, such as topography, hydrology, vegetation and soil, were created using remote sensing data (airborne LiDAR, ALOS/AVNIR-2, and ALOS/PALSAR). Using these geographical parameters as explanatory variables, the causes for Sasa expansion were analyzed by multiple linear regression analysis. It was shown that distance to natural ditches, gradient of ground surface, elevation, and carbon content are the largest contributors and that the hydrological factor is the one most associated with Sasa expansion. By using the landscape approach, 60% of the Sasa expansion factors could be explained. The analysis of the vegetation near the Sasa frontlines showed that the dynamics of sedge vegetation may serve as an indicator of potential Sasa expansion. Keyword: mire, dwarf bamboo, Sasa expansion, geographical parameter, remote sensing INTRODUCTION Wetlands/peatlands are unique ecosystems that form under specific environmental conditions, whereby high precipitation, and low temperatures result in low evaporation and waterlogged soils. Within these soils, plant decomposition is suppressed by anaerobic and acidic conditions. Together, wetlands and peatlands account for approximately 3% of Japan’s total terrestrial area (Pfadenhauer et al. 1993). In addition to their role in local climate and flood control, they serve as habitats for valuable wildlife, playing an important role in biodiversity conservation; many wetlands are currently under protection. Furthermore, the amount of carbon sequestered in wetland peat soil is said to be equivalent 58 Journal of Landscape Ecology (2012), Vol: 5 / No. 1 to the carbon content of all terrestrial plants. In recent years, the role of wetlands as a carbon sink has been under the spotlight in the climate change issue (Gorham 1991; Maltby and Immirzi 1993). Wetlands, however, are subjected to environmental changes. In particular, ombrotrophic bogs are vulnerable to vegetation changes caused by human impact or natural factors. Moreover, invasion of mesic (non-wetland) vegetation is a serious threat to bogs because it causes degradation of wetland ecosystems and their biodiversity (Hobbs and Humphries 1995). Identifying the current status of, and factors for, vegetation change is necessary to determine how effective control or restoration measures may be undertaken to preserve the bogs from expansion of mesic vegetation. The factors that cause vegetation change include biological, physical, and chemical factors, which are interrelated, and often encompass a wide range of scale from the microenvironment to an ecosystem, such as a wetland. This prevents the efficient development and implementation of measures for managing mesic conditions within bogs. To date, many studies on plant invasion have concentrated on the control of individual species or have analyzed limited geographical areas. The results of these studies are not always satisfactory (Hobbs and Humphries 1995). Kellner and Halldin (2002) and Petrone et al. (2004) confirmed the need to identify the spatial distribution of environmental factors for an entire target area, to ensure adequate conservation and management of wetlands, whilst Rydin and Jeglum (2006) stated the importance of large-scale conservation approaches. Hobbs and Humphries (1995) found that the spatial and temporal dynamics of the expansion of invading populations is an important component for plant invasions. In this study, we analyzed the causes for the expansion of dwarf bamboo (Sasa palmata), an indicator of mesic vegetation in contrast to moist environments, into bogs by using a landscape approach. To develop effective management strategies, we assessed the geographical factors that are associated with, and contribute to, changes in wetland vegetation from a landscape perspective. Sasa sp. belong to family Poaceae and are distributed throughout Far East Asia, particularly in China, Korea, Japan, and Sakhalin. They mainly propagate by means of horizontal rhizomal growth. In Japan, Sasa sp. are widely distributed from lowland to alpine areas, and occasionally, they form dense communities in the grasslands and forest understoreys; this interferes with the regeneration of other plants (Narukawa and Yamamoto 2002; Ito and Hino 2007; Suyama et al. 2000). In the wetlands of Japan, dwarf bamboo is an indication of mesic conditions. In many of these lowland wetland areas, because of the creation of agricultural drainage ditches in the surrounding areas, groundwater levels have decreased. This has led to the expansion of dwarf bamboo, which has become a problem (Fujita 2007).” METHODS Study area We chose the 1,546-ha Kami-Sarobetsu area (elevation: 5–7 m) of the 6,658-ha Sarobetsu Mire (Fujita et al. 2009), Toyotomi town, in northern Hokkaido, Japan, as the study site (Fig. 1). 59 Journal of Landscape Ecology (2012), Vol: 5 / No. 1. aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa Fig. 1: Study area 45.1 N 141.7 E The percentages of land usage for Toyotomi town are 29.9% farmland, 31.1% forestry, 21.9% mire, and 0.6% residential land (http://www.town.toyotomi.hokkaido.jp/). For the period 1981–2010, the average temperature was 6.1°C, the average annual rainfall was 1072.5 mm, and the average maximum snow depth was 99 cm. (http://www.jma.go.jp/). The formation of peatlands commenced roughly 4000–5000 years ago, and the current peat depth is about 6 m (Sakaguchi et al. 1985; Ohira 1995). Sarobetsu mire originally covered about 16,000 ha, and because of conversion for agriculture and peat extraction, ca 70% of the mire area has been lost (Hotes et al. 2010). The mire contains the largest bog in the country and is designated as a national park and a Ramsar site. The Kami-Sarobetsu area is surrounded by farmland (pasture land), and straightened rivers, drainage ditches and shortcut channels run along the boundary between the mire and farmland; these factors have resulted in low groundwater levels and drying of vegetation (Fujita et al. 2003). Sasa, which grows in a mesic environment, is distributed in the western half of the study site (Fig. 2, 3). It has expanded its distribution every year toward the bog vegetation in the east where Sphagnum moss is dominant; this expansion has threatened the native ecosystem of the area (Takagi et al. 1999; Ito and Wolejko 1990). Although the Japanese Ministry of the Environment and the Hokkaido Regional Development Bureau are (among other measures) implementing measures to mitigate for the negative effects of draining ditches, effective assessment of and countermeasures against Sasa expansion in the inner mire area have not been undertaken. In this study, the continuous frontlines of Sasa in the central area of the Kami-Sarobetsu mire were targeted. 60 Journal of Landscape Ecology (2012), Vol: 5 / No. 1 Fig. 2: Dwarf bamboo (Sasa palmata) Fig. 3: Sasa flontline (Dense coverage is 90% here) (Kami-Sarobetsu area: by Masayuki TAKADA) Parameter of Sasa expansion Fujita et al. (2003) reported Sasa expansion in the study area. They analyzed aerial photographs taken in 1977 and 2000 to determine the Sasa frontlines, which were then mapped using GIS. These data were used to determine the area expanded by Sasa during the past 23 years (Fig. 4). Fig. 4 Sasa frontline in the study area (Gray line: 1977, Black line: 2000, both from aerial photographs) Bog Sasa 500m 61 Journal of Landscape Ecology (2012), Vol: 5 / No. 1. aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa CREATION OF GEOGRAPHICAL PARAMETERS To analyze the factors that influence Sasa expansion on a landscape scale, distribution maps associated with topography, hydrology, vegetation and soil are necessary; therefore, using remote sensing and GIS data, geographical data was created for each environmental factor by using the following methods: Topography Several parameters were created using airborne LiDAR data (Ministry of the Environment and AeroAsahi, 2003.5.20; beam divergence: 0.2 m rad; footprint: 0.18 m; data resolution (grid size): 1 m). The elevation was set as the minimum value within the 3 m × 3 m area around a grid, and the ground surface gradient was calculated from elevation and set as the average value in the 10 m × 10 m area around a grid. Furthermore, the amount of ground subsidence that occurred in the past 50 years was determined by subtracting the current elevation level from the past elevation